For environmental protection, there has been a demand for compact batteries with high output density as the power source for vehicles and cellular phones. Specifically, a lithium ion battery, which includes an active material having high output density, has been drawing attention.
The application to the power source for vehicles requires a plurality of lithium ion batteries that are connected in series to increase the output power. However, batteries connected through connection members have a problem of power reduction due to electric resistance of the connection members. In addition, the connection member has disadvantageous effects on the space and weight of batteries.
In order to solve the problems, a bipolar battery has been developed. The bipolar battery includes a current collector having both faces on which a positive electrode active material and a negative electrode active material are provided.
The current collector that is one of the members of a bipolar battery and is in contact with a negative electrode is required to have stability in an equilibrium potential environment between the negative electrode active material and a lithium ion and electric conductivity. In addition, the current collector is required to pass no component contained in an electrolytic solution because the leakage of components in an electrolytic solution out of a system degrades the performance of a battery.
Patent Document 1 discloses using metal foil as a current collector and Patent Documents 2 and 3 disclose that a current collector containing a polymer material achieves weight reduction of the current collector and this improves the output density per weight of a battery.
However, in consideration of the structure of a bipolar battery, the positive electrode active material layer-forming face of the current collector is required to be stable in the equilibrium potential environment between the positive electrode active material and a lithium ion, and the opposite side, that is, the negative electrode active material layer-forming face of the current collector is required to be stable in the equilibrium potential environment between the negative electrode active material and a lithium ion.
In the case of a current collector using metal foil as disclosed in Patent Document 1, a current collector selecting a metal foil such as a SUS foil is stable in the equilibrium potential environments in both a positive electrode and a negative electrode, but a battery including such a collector is heavier than a battery including a current collector containing a polymer material as disclosed in Patent Documents 2 and 3, and this limits the improvement in output density.
In contrast, the current collectors disclosed in Patent Documents 2 and 3 reduce the weight of a battery and this should improve the output density. However, the current collector has insufficient stability in the equilibrium potential environments in both the positive electrode and the negative electrode and this specifically shortens battery life and is required to be improved. In addition, a current collector using a polymer material such as polyethylene and polypropylene passes a solvent of an electrolytic solution. On this account, a charged battery, in which a solvated lithium ion moves through the current collector, has a problem of failing to maintain the state of charge for a long period of time.
Patent Document 4 has studied a conductive resin film containing a cyclic olefinic resin and a conductive filler as a current collector of an electric double layer capacitor. However, the study whether the conductive resin film can be used as a current collector for a bipolar battery or not has revealed that the film has insufficient stability in the equilibrium potential environment in the positive electrode.